Epigenetic changes alter chromatin structure, thereby regulating gene transcription. In normal cells, repetitive DNA is hypermethylated and transcriptionally silent, whereas transcribed gene promoters are undermethylated and associated with open chromatin. Cancer cells are characterized by abnormal DNA methylation: repetitive DNA sequences and some gene promoters are hypomethylated and transcriptionally active, whereas many tumor suppressor gene promoters are hypermethylated and transcriptionally inactive. Although many studies have focused on categorizing which genes have altered DNA methylation patterns in cancer cells, the precise components of the DNA methylation machinery mediating those changes have not been established. We propose to develop a unique method of chemical cross-linking and protein complex identification to identify the factors involved in promoter hypermethylation in breast cancer. Our new strategy is based on the development of novel chemical compounds synthesized within the He Laboratory and has significant advantages over other approaches in that it assembles a protein complex directly on a specific biologically relevant DNA. We envision applying this strategy to determine a quantitative molecular signature for DNA methylating complexes in cancer cells. To develop our new technology, we propose focusing on determining the protein complexes that mediate the hypermethylation of the promoters of BRCA1, a classic tumor suppressor gene, and CDH1, which encodes a protein important for cell adhesion, using two Specific Aims: (1) To incorporate novel chemical cross-linking compounds into oligonucleotides corresponding to the BRCA1 and CDH1 promoters, perform cross-linking to protein extracts from breast cancer cells, and identify the cross-linked proteins by mass spectroscopy;and (2) To compare the DNA methylating complexes quantitatively in human breast tumors versus normal tissue using our technique. In the future, a detailed understanding of the DNMT/other protein contacts at particular gene promoters could lead to the development of hypomethylating agents targeted to these promoters in a sequence-specific manner, thereby avoiding the consequences of genome-wide hypomethylation. In addition, the new chemistry allows formation of DNMT-DNA complexes at high efficiency, which could facilitate the structural characterization of human DNMT-DNA complexes.